1. Field of the Invention
The present invention relates generally to a control system for controlling a power supply for an electric motor. More specifically, the invention relates to a fail-safe system in a drive control system of an electric motor, such as an induction motor. Further particularly, the invention relates to a fail-safe system which detects abnormality in a regenerated power absorbing circuit and thus detects abnormality in a power converter for performing fail-safe operation.
2. Description of the Background Art
British Patent First (unexamined) publication 2,167,252, published on May 21, 1986, discloses an elevator control system including an elevator hoist motor. The disclosed system also includes a drive circuit for the elevator hoist motor, which circuit has three phase alternating current rectifying converter. A regenerated energy absorbing circuit is connected across direct current (DC) terminals of the converter and comprises a regenerated energy absorbing or consuming resistor and a switch which comprises a switching transistor. The switch closes to permit the regenerated power to flow through the resistor for absorption thereof while the motor is in the operation of regenerative mode.
Generally, the regenerated power absorbing resistor tends to burn out in a short period when subjected to excess voltage exceeding its a rated voltage. Such excess voltage may be applied to the regenerated power absorbing resistor upon failure in a circuit component, such as the converter, for example. Breakage of the regenerated power absorbing resistor also tends to occur when the frequency of acceleration and deceleration becomes higher than a predetermined value. In such case, the regenerated power to be consumed by the regenerated power absorbing resistor becomes greater than the capacity of the resistor. As a result, the resistor tends to become over-heated or in the worst case broken. Furthermore, failure of the switch, i.e. switching transistor, also results in breakage of the resistor.
Once the breakage of the regenerated power absorbing resistor occurs, the regenerated energy of the motor drive circuit cannot be consumed which causes the current voltage in the circuitry to rise. This results in stoppage of an inverter via an excess voltage trip circuit. This makes it impossible to accurately control the position of a cage of the elevator. In such emergency situation, it is usual to generate an alarm and stop the motor. When this occurs, the cage may stop at an intermediate position trapping any passangers who may be in the cage.